Comparison of the Prevalences and Diversities of Listeria Species and Listeria monocytogenes in an Urban and a Rural Agricultural Watershed

Abstract

Foods and related processing environments are commonly contaminated with the pathogenic Listeria monocytogenes. To investigate potential environmental reservoirs of Listeria spp. and L. monocytogenes, surface water and point source pollution samples from an urban and a rural municipal water supply watershed in Nova Scotia, Canada, were examined over 18 months. Presumptive Listeria spp. were cultured from 72 and 35% of rural and urban water samples, respectively, with 24% of the positive samples containing two or three different Listeria spp. The L. innocua (56%) and L. welshimeri (43%) groups were predominant in the rural and urban watersheds, respectively. Analysis by the TaqMan assay showed a significantly (P < 0.05) higher prevalence of L. monocytogenes of 62% versus 17% by the culture-based method. Both methods revealed higher prevalences in the rural watershed and during the fall and winter seasons. Elevated Escherichia coli (≥ 100 CFU/100 ml) levels were not associated with the pathogen regardless of the detection method. Isolation of Listeria spp. were associated with 70 times higher odds of isolating L. monocytogenes (odds ratio = 70; P < 0.001). Serogroup IIa was predominant (67.7%) among the 285 L. monocytogenes isolates, followed by IVb (16.1%), IIb (15.8%), and IIc (0.4%). L. monocytogenes was detected in cow feces and raw sewage but not in septic tank samples. Pulsotyping of representative water (n = 54) and local human (n = 19) isolates suggested genetic similarities among some environmental and human L. monocytogenes isolates. In conclusion, temperate surface waters contain a diverse Listeria species population and could be a potential reservoir for L. monocytogenes, especially in rural agricultural watersheds.

FIG 1

Middle Musquodoboit watershed in Canada and sampling locations (MR1 to MR5). (Map created using ArcGIS v10.2.)

FIG 2

Lake Fletcher/Collin's Park watershed system and sampling locations (LF1 to LF6). (Map created using ArcGIS v10.2.)

FIG 3

UPGMA clustering analysis of human L. monocytogenes isolate AscI PFGE patterns and selected L. monocytogenes watershed isolates using the Dice coefficient with a tolerance of 1.5%. L. monocytogenes NS human food-borne outbreak isolates are labeled “human,” MR L. monocytogenes isolates are labeled “rural river,” and LF isolates are labeled “urban stream.”

FIG 4

UPGMA clustering analysis of human L. monocytogenes isolate ApaI PFGE patterns and selected L. monocytogenes watershed isolates using the Dice coefficient with a tolerance of 1.5%. L. monocytogenes NS human food-borne outbreak isolates are labeled “human,” MR L. monocytogenes isolates are labeled “rural river,” and LF isolates are labeled “urban stream.”

FIG 5

AscI multidimensional scaling in three dimensions in the coordinate space (a), followed by ApaI multidimensional scaling in three dimensions (b). Samples from the rural watershed (MR) are green/yellow, while samples from the urban watershed (LF) are represented by purple symbols. NS human food-borne outbreak isolates are indicated in red. The proportion of the data variance (r²) accounted for by MDS was 0.72.